1 April Monument

The gate tower at Bastion IX is an open timber structure with observation platforms, built on top of historical foundations of the city wall of Brielle. It serves to commemorate the 450th anniversary of the city’s liberation from the Spanish. The structure is made from specially modified wood for durability, digitally fabricated with high precision, and assembled using a unique combination of traditional carpentry joints, conventional fasteners and steel plates and frames. Summum Engineering provided the structural and detail design and engineering of the tower, taking the preliminary design all the way to final construction.

During the Eighty Years’ War between the Netherlands and Spain, the city of Brielle was retaken by Protestant rebels, the Watergeuzen, after they marched through the North Gate at Bastion IX on April 1, 1572. It marked a turning point in the conflict, as it was the first city to be liberated from the Spanish, and allowed for other towns to openly support William of Orange and oppose the Spanish Duke Alva. This day is still celebrated each year on April 1st and the night before throughout Brielle, as the residents dress up in 16th century attire. They reenact the events, organized by the 1-Aprilvereniging, which take place at several locations including the ruins at Bastion IX. To commemorate the 450th anniversary of the liberation of Brielle, the municipality of Brielle, now Voorne aan Zee, commissioned a new timber gate tower to be built on top of the old foundations. The tower not only functions as a stage but also revitalizes and opens up the previously overlooked site, allowing the public to engage with its historical significance.


In the Summer of 2018, ideas for Bastion IX were collected via an open call and creative session with an advisory group, local residents, businesses and organizations. The results included the concept of rebuilding the gate tower. RO&AD Architecten then developed several ideas, and presented them to the selection committee, advisory group and local residents in November 2019 for feedback. The final main concept was presented and exhibited in 2020.


The final design is an abstract copy of the North Gate without the city walls. The size of the gate was assumed based on archival, building history and archaeological research. The grid lines are derived from the corners in the outlines of the old tower foundations, producing irregular spacings between the wooden elements. Large diagonals form a large X through the orthogonal structure, and double as wind bracing and stringers for the staircases. The lower stairs take the visitors up to an observation deck, via a central suspended platform in the heart of the tower.

By subtly adding light, sound and texture to the timber structure of the gate tower, further information about the site’s history and relevance is transferred to its visitors. In addition, a line has been included in the final design that marks the approach route of the Geuzen in 1572. By creating intermediate levels in the surrounding levee walls, a theater setting is created, which can be used for events or as a place to stay.


Understandably, there was concern that the proposed plan would negatively affect the historical value of the site. Various studies were conducted, including archaeological surveys and excavations, to inform the final design of the tower. These plans were reviewed and ultimately approved by the Rijksdienst voor het Cultureel Erfgoed (RCE, Cultural Heritage Agency of the Netherlands).


The exposed, irregular structure is made of Accoya timber, which is wood that is modified by acetylation in order to improve its durability for outdoor conditions. The wood was CNC fabricated with high precision, in order to produce the various joints for assembly on site. The entire structure is connected using: carpentry joints such as mortise-and-tenon, dado and dovetail joints; steel fasteners such as bolts, screws and glued-in anchors; and unique steel gusset plates. At the base the tower connects to six steel frames, each in turn connected to vertical grout anchors, to prevent the uplift and overturning of the wooden tower. The anchors made use of existing holes in the historical foundations, that were drilled for archaeological and geotechnical survey. The impact on the historical site is extremely minimal, in part because the very top dates only to the 1970s, but also because the tower is lightweight, and could be disassembled in the distant future.


The unique geometry of the tower, though simple in appearance, is surprisingly complex due to the irregular spacings in the grid. Nearly every joint is unique, requiring attention during detailed design and engineering. To deal with the complicated geometry, a three-dimensional structural model was generated directly from the fabrication model using visual programming. Early on, the cross-sectional sizes were optimized to produce a sleek and elegant appearance, despite the use of wood. The final structural model includes scissor hinges and springs to model the timber connections and their eccentricities. The model includes specific guidance and material properties for the Accoya wood as well as wind loads from Dutch guidelines for scaffolded structures.

Digital fabrication

The design, featuring more than five thousand joints, was modeled and rationalized for fabrication by Geometria, in close collaboration and with guidance from Summum. They exported the 1833 unique Accoya timber pieces using custom-built file exporter, which made it easy to accurately export the hundreds of unique timber pieces directly for fabrication to the Hundegger timber CNC machines. These parts were fabricated by TimberFrame Oy in Kuhmo, Finland, with a fraction of a millimeter accuracy.


Van Hese Infra took on the final assembly, akin to solving a giant puzzle at the site. Geometria supplied a large number of assembly and construction drawings for the project. Furthermore, builders could use the detailed 3D model on-site, providing crucial support in constructing this three-dimensional complex monument.


The timber structure incorporates a sound feature and text engravings to convey additional insights into the site’s history. Night-time illumination enhances the visual aesthetics and highlights the complexity of the beam structure. Kinkorn applied stainless steel letters on the interior, writing out the chorus of the song “In the name of Orange, open the gate!”. The song is also heard and triggered upon entering the gate. Overall, the 1 April Monument in Brielle is an intricately designed and historically significant timber tower accessible to the public. It offers the same views of the area as those 450 years ago.


Gemeente Brielle

Design team
Ro Koster, Ad Kil, Martin van Overveld, Bram Franken, Youp van Terheijden | RO&AD Architecten (architect)
Toni Österlund, Lisa Voigtländer | Geometria (digital fabrication)
Diederik Veenendaal | Summum Engineering (structural engineering)

Exhibition design

Diederik Veenendaal | Summum Engineering (final and construction design)
Carla Mulder, Edwin Swierstra | Royal HaskoningDHV (geotechnical engineering consultant)
Edwin Vlasblom | Fugro (geotechnical engineering)
Hendrik-Jan Cassee, Remco van Osch | Nebest (survey historical foundation)

Van Hese Infra (general)
Van Leeuwen (foundation)

Oy Timber Frame (CNC production)
Kloosterboer (lettering)

Accoya (timber)
Carl Stahl (steel mesh)

ADC ArcheoProjecten (survey)

Location: Brielle, Netherlands
Time: 2020-2022 (design and engineering), 2022-2023 (construction)
Status: Completed
Size: 11.5 x 10 x 10m
Services: Structural engineering, detailed engineering


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